fix(serial): don't sleep forever with pending PhoneAPI output on UART consoles (#11500)

* fix(serial): don't sleep forever with pending PhoneAPI output on UART consoles

Since #11164 bounded the stream drain, a config dump can end a dispatch
with output still queued. On UART-console ESP32 boards runOnce() then
returns INT32_MAX with no RX pending, and neither rxInt() nor
onNowHasData() fires for the remaining output, so the download wedges
mid nodeinfo stream until the client happens to send a byte.

Add StreamAPI::hasPendingOutput() (transport-retained frame or queued
PhoneAPI data) and have SerialConsole::runOnce() short-poll (<=25ms)
while it holds instead of sleeping INT32_MAX. The #11164 write budget
is unchanged; idle sleep behavior with a drained queue is unchanged.
The retained-frame probe also covers the ESP32-S2 USB-CDC branch,
which takes the same INT32_MAX path.

* test(serial): restore scratch NodeDB via tearDown, trim comments to house style

A failed TEST_ASSERT longjmps out of a Unity test without running
destructors, so RAII cannot restore the swapped nodeDB pointer; install
the scratch NodeDB explicitly and restore/delete it in tearDown(),
which runs after every test outcome. Also shorten the new comments to
the two-line house limit.

src/SerialConsole.h

@@ -51,6 +51,8 @@ class SerialConsole : public StreamAPI, public RedirectablePrint, private concur
 
     /// Continue retained USB CDC output before PhoneAPI advances.
     virtual bool finishPendingFrame() override;
+    /// Report a retained USB CDC frame awaiting TX space.
+    virtual bool hasRetainedFrame() override;
     /// Return whether the dedicated log buffer can be safely overwritten.
     virtual bool canEncodeLogRecord() override;
     /// Write or retain one framed USB CDC message.